The present invention relates to a slip-controlled brake system for road vehicles with all-wheel drive. In order to compensate the differences in distances the separate wheels have to cover, these all-wheel drive vehicles are equipped with differential gears or so-called differentials. The compensating effect of the differentials being adapted to be neutralized or reduced by virtue of differential locks which operatively or positively couple the output shafts.
Known vehicles with all-wheel drive are equipped with both a front-axle differential and a rear-axle differential, as well as with an intermediate differential which interconnects the front-axle drive and the rear-axle drive. To improve the driving behavior and force transmission in the event of unfavorable road conditions, in particular on ice and snow or if the road is slippery on one side thereof, it is known to mount locks into the intermediate and the rear-wheel differential which permit to partial or complete neutralization of the compensating effect by coupling of the output shafts. As a general rule, the front-axle differential does not include such a lock because this would excessively impair steerability.
In vehicles with slip control, however, the rigid coupling of the driven wheels and axles will cause difficulties after the differential locks are put into operation. This is because the control unit which (in consideration of the wheel rotation and the changes thereof) determines the necessary further braking pressure variation for the purpose of accomplishing an optimal braking and driving behavior, is not able to distinguish between the torque variations transmitted by the rigid coupling and those torque variations resulting from the control of the brake force and the friction force between the tire and road. For this reason, it has been necessary to de-activate brake slip control when engaging the differential locks, this bearing the disadvantage that, especially in the event of unfavorable road or weather conditions when the differential locks are useful, one had to do without brake slip control. This is a disadvantage because slip control could accentuate its special advantages on snow and slippery roads, in the presence of different friction values on the right/left side and the like, and could preserve in particular steerability and the driving stability of the vehicle.
It is therefore the object of the present invention to overcome the shortcomings of the prior art and to develop a slip-controlled brake system which is particularly suitable for road vehicles with all-wheel drive.